Corrosion of steel in reinforced concrete is a major problem. Both sustained and temporary electrochemical treatments have been used to arrest this problem. These involve passing a current through the concrete to the steel from an installed anode system. In all cases the steel becomes the cathode of the electrochemical cell that is formed. In impressed current electrochemical treatment, the anode is connected to the positive terminal and the steel is connected to the negative terminal of a source of DC power. In sacrificial electrochemical treatment, the protection current is provided by corroding sacrificial anodes that are directly connected to the steel.
Sustained or long term electrochemical treatments are installed with the intention of maintaining the treatment for the foreseeable future. The electrochemical treatment period would typically be measured in years. A well known family of sustained or long term techniques is cathodic protection. It includes impressed current cathodic protection, sacrificial cathodic protection, intermittent cathodic protection and cathodic prevention. In these techniques a long term or permanent anode delivers a small current to the steel reinforcement. Average current densities expressed per unit area of steel surface typically range from 2 to 20 mA/m2 to arrest existing deterioration and 0.2 to 2 mA/m2 to prevent the initiation of deterioration. The current may be pulsed but the average applied currents are typically within the above ranges. The current may from time to time be adjusted with adjustments based on an analysis of performance data.
Temporary or short term electrochemical treatments are installed with the intention of discontinuing the treatment in the foreseeable future. The electrochemical treatment period would typically be measured in days, weeks or months. Temporary treatments designed to arrest reinforcement corrosion include chloride extraction (U.S. Pat. No. 6,027,633) and re-alkalisation (U.S. Pat. No. 6,258,236). In these systems a temporarily installed anode system is used in conjunction with a temporary DC power supply to deliver a large current of the order of 1000 mA/m2 expressed per unit area of steel surface for a short period (typically less than 3 months) to the steel reinforcement.
Anodes are electrodes supporting a net oxidation process. Anodes for concrete structures may be divided into inert anodes or sacrificial anodes. They may be further divided into anodes that are embedded within a porous matrix or anodes that are attached to the concrete surface such that they are exposed and accessible, as well as into discrete or non-discrete anodes. Anode systems that include an anode and a supporting electrolyte may be divided into temporary and long term anode systems. A summary of the differences is given in the following paragraphs.
Inert anodes resist anode consumption. They have been used in most electrochemical treatments, the principle exception being sacrificial cathodic protection. The main anodic reaction is the oxidation of water producing oxygen gas and acid. The acid attacks the cement paste in concrete. As a result, the current density off inert anodes tends to be limited to less than 200 mA/m2 expressed per unit area of anode surface. A widely used anode system is a mixed metal oxide (MMO) coated titanium mesh embedded in a cementitious overlay on the concrete surface (U.S. Pat. No. 5,421,968). A discrete porous titanium oxide anode that is claimed to deliver higher anode current densities up to 1000 mA/m2 off the anode surface has also been used (U.S. Pat. No. 6,332,971).
Sacrificial anodes are consumed in the process of delivering the protection current. The main anodic reaction is the dissolution of the sacrificial metal. As a result the life of sacrificial anodes is limited. Sacrificial anodes have been applied as embedded (buried) discrete anodes in sacrificial cathodic prevention systems (WO 9429496) and as a mesh with an overlay in sacrificial cathodic protection (U.S. Pat. No. 5,714,045). However the use of embedded sacrificial anode systems is deterred by the need to replace the anodes at the end of their life. Sacrificial anodes systems have also been attached directly to the concrete surface (U.S. Pat. No. 5,650,060) and are accessible to facilitate anode replacement. The use of sacrificial anodes in an impressed current role is deterred by the more rapid consumption of the anode in this role. However surface applied anodes are easy to replace and this type of system has been used as an impressed current anode at anode current densities typically less than 25 mA/m2 (U.S. Pat. No. 5,292,411).
Discrete anodes are individually distinct compact anodes that are normally embedded in holes in the concrete or installed at locations where patch repairs to the concrete are undertaken. A description of discrete anodes is given in U.S. Pat. No. 6,217,742. Embedded discrete anodes are strongly attached to the concrete and attachment failures are less common for discrete anodes than for the non-discrete anodes applied to concrete surfaces.
Temporary anode systems are usually attached to the concrete surface to deliver short term high current temporary electrochemical treatments and are removed at the end of the treatment period that is typically less than 3 months. Temporary anodes are surrounded by a temporary electrolyte, such as a liquid contained in a tank or an electrolytic material such as saturated cellulose fibre, that is easily removed at the end of the treatment process (U.S. Pat. No. 5,538,619). A high drive voltage together with a high volume of electrolyte is generally needed to support the high current output. By contrast, long term anode systems, intended to deliver a protection current over several years, are strongly attached to the concrete and may be embedded in cavities in the concrete to improve anode attachment.